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Testes Size Correlates with Men’s Involvement in Toddler Care

Testes sizeThere are good men, there are bad men. People usually use men’s involvement in toddler care as a standard to determine good husbands from bad ones. A family-oriented man is good at taking care of children, can bathe children, bring children to doctor and soothe children to sleep and even has stronger brain reaction when sees photos of his children – a new study says that this kind of family-oriented men usually have small testes.

Human, as a creature, also takes reproduction as the top priority among all the activities and task. But how can human get as many offspring as possible? There exist two ways: the first one is extensive cultivation, while the other one is intensive cultivation. The extensive cultivation requires putting all the “recourses” into plenty of sperms, while the intensive cultivation calls for spending effort on the children already born. It can be seen easily that these two approaches are contradict with each other.

Jennifer S. Mascaro and colleagues from Emory University, proved the existence of this contradiction using functional magnetic resonance imaging (fMRI) technology. The researchers selected 70 biological fathers who had a child between 1 and 2 and showed them with photos of their children while measuring their brain activity. It was discovered that those who had smaller testes volume showed stronger brain activity upon seeing their children.

One of the researchers, James Rilling noted that similar phenomenon has been observed in other species:” In primates, monogamous animals such as titi, has smaller testes volume than promiscuous animals like chimpanzee. For sheep, individuals with larger testes also mate more than those with smaller testes.”

The researchers also investigated the participants with survey and the survey revealed that subject participants with smaller testes volume claimed to have more involvement in toddler care.

However, the author pointed out that although statistically significant, the correlation between testes size and caregiving was not perfect and the two were not proven to have casualty. The difference in testes size can only explain around 20 percent of caregiving activities. Also, it is possible that these men were willing to be more hands-on fathers and thus resulted in smaller testes size. This study needs to be further investigated, though, it did indicate that biological factors play roles in caregiving activities.

This study has been published on recent PNAS.